Radev, who has called for an end to European Union sanctions against Russia, won nearly 60 percent of the vote, according to partial official results.

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Radev's victory over pro-EU ruling party candidate Tsetska Tsacheva and the collapse of Borisov's minority government is likely to result in months of political uncertainty - and bring the country politically closer to Russia.

"It's a victory for all Bulgarian people. Democracy has beaten apathy and fear today," Radev told public broadcaster BNT.

Radev, a 53-year-old jet fighter pilot and novice to politics, tapped into public anger with political elites and fears about immigration, and vowed not to make the Balkan country a "migrant ghetto".

The country of 7.2 million people already has warmer ties with Russia than most of its EU peers.

Radev has made it clear he believes NATO member Bulgaria should focus more on its economic and political ties with Moscow, which has been under EU sanctions since it annexed Crimea from Ukraine in 2014.

Under Bulgaria's constitution, the president's job is mostly ceremonial, but whoever holds the post can influence policy, veto legislation and sign international treaties.